The disclosure relates to an optical device, which is preferred in the case of transmitting light in multiple modes.
Optical fibers for use in widely available optical fiber communication systems have a structure in which the outer circumferential surface of one core is surrounded by a cladding. Optical signals propagate through the inside of this core, and thus information is transmitted. In these years, with the wide spread use of optical fiber communication systems, information volumes to be transmitted are dramatically increased.
In order to achieve an increase in the transmission capacity of such optical fiber communication systems, there is known a method in which a multicore fiber includes a plurality of cores and a cladding surrounding the outer circumferential surface of the cores and this multicore fiber is used to transmit a plurality of signals with light propagating through each core.
For example, Patent Literature 1 below discloses an optical device for transmitting light beams to and from a multicore fiber. In the optical device, a single-core optical fiber is inserted into each of a plurality of holes formed in a capillary to form an assembly, the assembly is drawn, and then the optical device is formed. The optical fiber is tapered in diameter from a first portion to a second portion of the optical fiber. Each of the optical fibers has a dual structure in which a core is formed of an inner core and an outer core surrounding the inner core with no gap. The refractive index of the inner core is set higher than the refractive index of the outer core. The refractive index of the cladding is set lower than the refractive index of the outer core. In the first portion, which is not tapered in diameter, the diameter of the inner core and the refractive index difference between the inner core and the outer core are set so that a single-mode light beam easily propagates through the inner core. In the second portion, which is tapered in diameter, the outer diameter of the outer core and the refractive index difference between the outer core and the cladding are set so that a single-mode light beam easily spreads to the outer core and propagates through the entire core including the inner core and the outer core.
In the optical device, a single-mode light beam propagating from the first portion to the second portion of each of the optical fibers propagates through the inner core in the first portion. However, the diameter of the inner core is below a predetermined diameter at the tapered second portion, and the leakage of light from the inner core to the outer core is increased. Consequently, after the diameter of the inner core is below a predetermined diameter, the light beam spreads to the outer core, and propagates through the core including the inner core and the outer core. Thus, in the optical device, although the pitch between the cores is decreased in the tapered second portion, a change in the mode field diameter (MFD) of the light beam propagating through each of the cores is decreased in the first portion and the tapered second portion.
[Patent Literature 1] JP2013-522677 A